How Many Gallons Is My Fish Tank?
Knowing the precise water volume of your aquarium is the foundation of good fishkeeping. It determines how many fish you can safely keep, how much dechlorinator and medication to dose, what filter turnover rate you need, and how much water to change each week. Our aquarium tank volume calculator handles seven common tank shapes — including cylinder, sphere, hexagon, corner, and L-shape — with support for glass thickness deductions and usable water estimates. All results display in both gallons and litres, with every measurement shown in both inches and centimetres.
How to Use the Tank Volume Calculator
The calculator updates instantly as you type — no submit button needed. Here is a step-by-step walkthrough:
Select your tank shape
Choose from rectangular, bow front, cylinder, sphere, hexagon, corner pentagon, or L-shape. The input fields update automatically to match the dimensions each shape requires.
Choose your input unit
Toggle between Inches and Centimetres. Results always display in both gallons and litres, and all measurements show both imperial and metric regardless of which input unit you choose.
Enter your dimensions
Measure the outside of your tank with a tape measure. If you only have inner dimensions, set glass thickness to "Not included — measuring inside" to avoid double-subtracting the panels.
Set glass thickness
Select from presets ranging from 5 mm (nano tanks) to 19 mm (very large). The calculator subtracts this from length, width, and height (×2 per axis) to give the true interior volume. Thicker glass on larger tanks can reduce volume by several gallons.
Adjust deduction sliders
Set your substrate depth, headspace at the top, and percentages for decorations and equipment. The usable water figure accounts for all of these — use it for medication dosing and bioload calculations.
Read your results
The results panel shows gross volume (gal + L), usable water, filled tank weight (lbs + kg), and your 25% weekly water change amount. Every dimension readout shows both inches and centimetres simultaneously.
Volume Formulas for Every Tank Shape
All calculations work in cubic inches first, then convert to your chosen output unit. To convert cubic inches to US gallons, divide by 231. To get litres from cubic inches, multiply by 0.016387.
Rectangular tank
The most common shape. Multiply all three interior dimensions together.
All dimensions in inches. Result in US gallons.
Bow front tank
The curved front panel reduces volume to approximately 87% of an equivalent rectangle.
W is the maximum width at the bow's widest point.
Cylinder tank
Uses the area of a circle (π × r²) multiplied by height.
D = diameter; H = height, both in inches.
Sphere / fish bowl
The formula for a perfect sphere — rarely used at full volume since fish bowls are typically only partially filled.
D = outer diameter in inches.
Regular hexagon tank
Hexagonal area uses the apothem formula for a regular 6-sided polygon.
S = length of one side; H = height, both in inches.
L-shape tank
Calculated as two overlapping rectangles. Measure both arms separately — the calculator handles the combination.
Ensure the two arm measurements do not overlap at the join.
Always measure the outside of your tank unless you can reach inside comfortably. Then use the glass thickness dropdown to subtract panel thickness from all dimensions. Measuring inside a filled tank is impractical — outside measurements plus glass deduction is the standard approach.
Aquarium Tank Volume Calculator
Enter your tank dimensions below. All results display in both gallons & litres, and every measurement shows both inches and centimetres simultaneously.
Standard Aquarium Sizes — Dimensions & Volume
Aquarium manufacturers use nominal sizes (10 gallon, 55 gallon, etc.) that are approximate. Actual volume varies slightly between brands, and the usable water volume is always less than the nominal rating once substrate, headspace, and decorations are accounted for.
| Nominal size | Exterior L × W × H | Gross volume | Usable water est. | Full weight est. | Best use |
|---|---|---|---|---|---|
| 2.5 gallon | 12 × 6 × 8"30 × 15 × 20 cm | 2.5 gal9.5 L | ~1.8 gal / ~7 L | ~27 lbs / ~12 kg | Shrimp / betta |
| 5 gallon | 16 × 8 × 10"41 × 20 × 25 cm | 5 gal19 L | ~3.8 gal / ~14 L | ~48 lbs / ~22 kg | Nano community |
| 10 gallon | 20 × 10 × 12"51 × 25 × 30 cm | 10 gal38 L | ~7.8 gal / ~30 L | ~94 lbs / ~43 kg | Beginner community |
| 20 gallon long | 30 × 12 × 12"76 × 30 × 30 cm | 20 gal76 L | ~15.8 gal / ~60 L | ~183 lbs / ~83 kg | Planted / community |
| 20 gallon high | 24 × 12 × 16"61 × 30 × 41 cm | 20 gal76 L | ~15.4 gal / ~58 L | ~181 lbs / ~82 kg | Display / tall plants |
| 29 gallon | 30 × 12 × 18"76 × 30 × 46 cm | 29 gal110 L | ~22.8 gal / ~86 L | ~260 lbs / ~118 kg | Community |
| 40 breeder | 36 × 18 × 16"91 × 46 × 41 cm | 40 gal151 L | ~31.5 gal / ~119 L | ~356 lbs / ~161 kg | Breeding / cichlids |
| 55 gallon | 48 × 13 × 21"122 × 33 × 53 cm | 55 gal208 L | ~43.5 gal / ~165 L | ~497 lbs / ~225 kg | Full community |
| 75 gallon | 48 × 18 × 21"122 × 46 × 53 cm | 75 gal284 L | ~60.5 gal / ~229 L | ~665 lbs / ~302 kg | Large community |
| 90 gallon | 48 × 18 × 24"122 × 46 × 61 cm | 90 gal341 L | ~72.8 gal / ~276 L | ~788 lbs / ~357 kg | Cichlids / reef start |
| 125 gallon | 72 × 18 × 22"183 × 46 × 56 cm | 125 gal473 L | ~101 gal / ~382 L | ~1,085 lbs / ~492 kg | Large display |
| 180 gallon | 72 × 24 × 25"183 × 61 × 64 cm | 180 gal681 L | ~147 gal / ~556 L | ~1,540 lbs / ~699 kg | Reinforce floor |
Tanks over 75 gallons (284 L) typically weigh more than 600 lbs (272 kg) when full. Always verify your floor's load-bearing capacity before filling a large aquarium — residential floors typically support 40–50 lbs per square foot. Consult a structural engineer for tanks over 100 gallons if your floor gives you any concern.
Aquarium Glass Thickness Guide
Glass thickness is selected by manufacturers based on the hydrostatic pressure the panel must withstand at a given height. Using glass that is too thin for a tall or large tank risks catastrophic failure. The table below shows standard glass thickness for common tank sizes.
| Glass thickness | Typical use | Max recommended height | Volume range |
|---|---|---|---|
| 5 mm (3/16") | Nano tanks, shallow displays | ~12" / 30 cm | 2–10 gallons / up to ~38 L |
| 6 mm (1/4") | Standard small tanks | ~18" / 46 cm | 10–29 gallons / 38–110 L |
| 8 mm (5/16") | Medium-small tanks | ~20" / 51 cm | 20–40 gallons / 76–151 L |
| 10 mm (3/8") | Standard medium tanks | ~22" / 56 cm | 30–75 gallons / 114–284 L |
| 12 mm (15/32") | Large tanks | ~24" / 61 cm | 75–125 gallons / 284–473 L |
| 16 mm (5/8") | Extra-large tanks | ~28" / 71 cm | 125–200 gallons / 473–757 L |
| 19 mm (3/4") | Very large / commercial | ~36" / 91 cm | 200+ gallons / 757+ L |
Acrylic tanks of the same volume can use thinner panels than glass — typically 0.4× the glass equivalent — but scratch far more easily. Many acrylic tanks also have slight optical distortion. For volume calculation purposes, measure and treat acrylic thickness the same way as glass.
How Tank Shape Affects Fishkeeping
Two tanks with the same nominal volume can have dramatically different surface area, swimming space, and suitability for different fish. Shape is one of the most under-considered factors when choosing an aquarium.
Rectangular (long)
The gold standard for fishkeeping. Maximises surface area for gas exchange and provides the most usable horizontal swimming space. The 40 breeder (36×18×16") is particularly popular for its wide, shallow footprint — ideal for bottom-dwellers, corydoras shoals, and planted tanks.
Rectangular (tall / high)
Same footprint as long tanks but more height. Looks dramatic and suits tall stem plants, discus, and angelfish. Less surface area per volume means lower oxygen capacity. Requires a more powerful filter to circulate water through the full water column.
Bow front
Curved front glass gives a wider viewing angle and a slightly larger interior depth. Volume is approximately 87% of an equivalent rectangle. Popular for living room display tanks. The curved front does make attaching external equipment slightly more challenging.
Cylinder
Visually stunning but less practical. Surface area is significantly lower than a rectangle of the same volume. Filtration is tricky — most HOB filters cannot attach to a curved rim. Best suited for nano displays or jellyfish tanks with circular flow systems. Volume ~78% of bounding rectangle.
Hexagon
A classic centrepiece shape. Provides a 360° viewing angle when positioned in the centre of a room. Volume is roughly 65% of the bounding rectangle. Surface area to volume ratio is lower than long tanks. Better suited to display purposes than high-density community stocking.
Corner & L-shape
Space-efficient designs that tuck into room corners or follow wall layouts. Both shapes provide good surface area. Volume calculations are more complex — use the calculator rather than estimating. L-shapes work well as dividing features in open-plan rooms.
How Much Does a Full Aquarium Weigh?
Water is extremely heavy — 8.34 lbs per US gallon (1 kg per litre). Most people significantly underestimate how much a filled aquarium weighs. The table below gives a quick sense of what to expect.
| Tank size | Water weight | Glass + stand est. | Total est. | Floor note |
|---|---|---|---|---|
| 10 gal / 38 L | ~83 lbs / ~38 kg | ~12 lbs / ~5 kg | ~95 lbs / ~43 kg | Standard floor |
| 20 gal / 76 L | ~167 lbs / ~76 kg | ~24 lbs / ~11 kg | ~191 lbs / ~87 kg | Standard floor |
| 40 gal / 151 L | ~334 lbs / ~151 kg | ~50 lbs / ~23 kg | ~384 lbs / ~174 kg | Standard floor |
| 55 gal / 208 L | ~459 lbs / ~208 kg | ~70 lbs / ~32 kg | ~529 lbs / ~240 kg | Standard floor |
| 75 gal / 284 L | ~626 lbs / ~284 kg | ~100 lbs / ~45 kg | ~726 lbs / ~329 kg | Check if joists span >12' |
| 125 gal / 473 L | ~1,043 lbs / ~473 kg | ~150 lbs / ~68 kg | ~1,193 lbs / ~541 kg | Verify floor capacity |
| 180 gal / 681 L | ~1,501 lbs / ~681 kg | ~220 lbs / ~100 kg | ~1,721 lbs / ~781 kg | Engineer assessment advised |
Position large aquariums perpendicular to floor joists where possible so the weight distributes across multiple joists. Running a tank parallel to a single joist concentrates all the load on one structural member. For tanks over 75 gallons on an upper floor, consult a structural engineer before installation.
Why Accurate Tank Volume Is Essential
Medication dosing
Most aquarium medications are dosed per gallon or per 10 gallons (or per 10 litres). An error of 20% in your volume estimate can result in significant under- or overdosing. Underdosing is ineffective; overdosing can be fatal to fish and invertebrates, particularly shrimp and scaleless species.
Water conditioner & dechlorinator
Dechlorinators like Prime are dosed by volume. During water changes you only treat the new water being added — knowing exactly how many gallons (or litres) you remove and replace lets you dose precisely and avoid wasting expensive conditioner.
Stocking limits
Bioload-based stocking calculations depend entirely on accurate water volume. Overestimating your tank volume by even 10 gallons (38 L) can result in significantly overstocking — with real consequences for water quality and fish health. Use the stocking calculator with the volume figure from this tool.
Filter and pump sizing
Filters are rated by flow rate (gallons per hour / litres per hour). The common guideline is to turn over tank volume 4–10× per hour. A 55-gallon (208 L) tank needs a filter rated for 220–550 GPH (833–2,082 LPH). Using gross vs. usable volume can shift this recommendation considerably.
Frequently Asked Questions
Answers to the most common aquarium volume questions.
How do I calculate the volume of a fish tank in gallons?
How many gallons is a 48×13×21 tank?
Does glass thickness affect the volume of an aquarium?
How much does a full fish tank weigh?
What is the difference between gross volume and usable water volume?
How do I convert fish tank gallons to litres?
How do I measure the volume of a hexagonal or cylinder fish tank?
Why does tank shape matter for fishkeeping?
What is the right tank size for a beginner?
How much water should I change each week?
Plan Your Aquarium with Our Stocking Calculator
Once you know the exact volume of your tank, the next step is working out how many fish it can safely support. Our Aquarium Stocking Calculator uses a bioload-based method covering 100+ freshwater, saltwater, and brackish species — with live compatibility warnings, school-size alerts, and filtration quality adjustments.
Use the volume figure from this tool as your tank size input in the stocking calculator for the most accurate results.